Now that I've read the full paper: they've have taken all the research novelty out of my sails. I'm amazed how parallel our designs are. I guess once you've gone down the rabbit hole of committing to operating on a dense serialized format, things like decoupling application data types from concrete representations suggest themselves. I'd be sunk if I were doing this for Science.
But I'm doing this for Engineering, and I like my implementation choices. I have achieved some properties that I think are important for practical integration with other code. The working name of my project is Re, because the core data structure is reified traversals and operating on them involves reifying some other things that aren't normally reified.
Edit to add: my elevator pitch (needs condensing, I haven't pitched it to anyone but my mom):
It used to be common practice to use tree structures to represent linear data. As CPUs have become faster than RAM the pointer-heaviness of this approach has made the constant factors awful, and it's now avoided when possible. It's now actually efficient to do the opposite and represent trees as linear data when you can. This can be done explicitly, e.g. in the WASM format, but such structures can't be operated on directly unless it's done "in the raw", outside the type system and other facilities of the language. What's missing is a generic abstraction layer that maps between operations on conceptual application objects and linearized trees. I've found that once you have that, you can also do things with linearized data that would be impractical with trees.
(Actually, it happened the other way around. Some time ago I prototyped a regex-like engine for finding and replacing fragments of ASTs [https://blog.lambdaverse.org/comacro/], and I discovered operating directly on serialized representation enabled that. I haven't had a chance to make a production implementation yet. Then recently I was working on tree-structured template inference to turn collections of webpages back into the databases that generated them with minimal interaction, and I realized... the comacro construction is a perfect fit here. So it became an excuse to develop that idea ostensibly as part of a project I could actually make money with.)